12.1 Fundamental Organometallic Reactions

Fundamental Organometallic Reactions

Organometallic reactions center on metal-carbon bonds and enable transformations that purely organic methods can't achieve. These reactions underpin the synthesis of pharmaceuticals, polymers, and industrial chemicals. The fundamental steps covered here, including oxidative addition, reductive elimination, insertion, and others, are the elementary moves that combine into catalytic cycles. Understanding each one individually is essential before you can follow how full catalytic mechanisms work.

Fundamental Reaction Steps

Oxidative Addition and Reductive Elimination

Oxidative addition increases both the oxidation state and coordination number of a metal center. The metal breaks a covalent bond in a substrate (like $\text{H–H}$, $\text{C–X}$, or $\text{H–X}$) and forms two new metal-ligand bonds. A classic example: a square planar $\text{Pt(II)}$ complex adds $\text{H}_2$ to become an octahedral $\text{Pt(IV)}$ dihydride. The metal formally goes from $d^8$ to $d^6$, losing two electrons to the new bonds.

Reductive elimination is the reverse. Two ligands on the metal combine to form a new covalent bond, and the metal's oxidation state and coordination number both decrease. This is the product-forming step in many catalytic cycles. For instance, reductive elimination of two organic fragments from a $\text{Pd(II)}$ center creates a new $\text{C–C}$ bond and regenerates $\text{Pd(0)}$.

These two reactions almost always appear as a matched pair in catalytic cycles. Oxidative addition activates the substrate at the start, and reductive elimination releases the product at the end. For oxidative addition to be favorable, the metal needs to be electron-rich and in a low oxidation state. For reductive elimination, the reverse applies: it's favored when the metal is in a higher oxidation state and the resulting bond is thermodynamically stable.

Insertion and Elimination Reactions

Migratory insertion combines a coordinated ligand with an adjacent ligand on the same metal. Two important types:

• 1,1-insertion (CO insertion): A coordinated $\text{CO}$ inserts into a metal-alkyl bond, forming an acyl complex ($\text{M–COCH}_3$ from $\text{M–CH}_3$ and $\text{CO}$). The migrating group actually moves to the CO, not the other way around, which is why it's called migratory insertion.
• 1,2-insertion (alkene insertion): An alkene inserts into a metal-hydride bond, forming a new metal-alkyl species. This is the chain-growth step in olefin polymerization.

β-Hydride elimination is the reverse of 1,2-insertion. A hydrogen on the carbon β to the metal transfers back to the metal, generating a metal hydride and a free alkene. Two requirements must be met: the metal needs a vacant coordination site, and the $\text{M–C}_\alpha\text{–C}_\beta\text{–H}$ unit must be able to adopt a coplanar arrangement (roughly 0° dihedral angle).

These steps are central to major industrial processes:

• Hydroformylation uses CO insertion into a metal-alkyl bond, preceded by alkene insertion into a metal-hydride bond, to convert alkenes into aldehydes.
• Ziegler-Natta polymerization relies on repeated 1,2-insertions of ethylene or propylene into a metal-alkyl bond to build polymer chains.

Transmetallation and Ligand Substitution

Transmetallation transfers an organic group from one metal center to another. In cross-coupling catalysis, this is how the organic fragment gets onto the catalytically active transition metal. For example, in a Suzuki coupling, an aryl group transfers from a boron "ate" complex to a $\text{Pd(II)}$ center. In a Stille coupling, the transfer comes from an organotin compound. The driving force is typically the formation of a more thermodynamically stable metal-heteroatom bond on the main-group side (e.g., $\text{B–O}$ or $\text{Sn–X}$).

Ligand substitution replaces one ligand with another at the metal center. Three mechanisms are possible:

• Dissociative (D): The departing ligand leaves first, creating an intermediate with a lower coordination number, then the incoming ligand binds. Favored for 18-electron complexes that are sterically crowded.
• Associative (A): The incoming ligand binds first, forming a higher-coordinate intermediate, then the departing ligand leaves. Favored for 16-electron complexes with available coordination sites.
• Interchange (I): Bond making and bond breaking happen concurrently in a single step, without a distinct intermediate.

Both transmetallation and ligand substitution tune the electronic and steric environment at the metal, which directly controls reactivity and selectivity in catalytic systems.

Addition Reactions

Nucleophilic and Electrophilic Additions

Nucleophilic addition involves a nucleophile attacking an unsaturated ligand that is coordinated to a metal. Coordination to the metal activates the ligand toward nucleophilic attack by withdrawing electron density from it. Common targets include coordinated $\text{CO}$, alkenes, and alkynes. For example, nucleophilic attack on a coordinated $\text{CO}$ can generate acyl or alkoxy-carbene complexes.

Electrophilic addition occurs when an electrophile attacks a metal-ligand bond that has nucleophilic character. Protonation of a metal-alkyl bond, for instance, can release an alkane through electrophilic cleavage. Halogenation of metal-alkene complexes can produce haloalkyl species.

These additions are how new organic functional groups get built in metal-mediated synthesis. Catalytic hydrogenation of alkenes, for example, involves nucleophilic delivery of a metal hydride to a coordinated olefin. Wacker-type oxidations use nucleophilic attack of water on a Pd-coordinated alkene.

Carbene Insertion and Related Processes

Carbene insertion involves a metal-carbene ($\text{M=CR}_2$) fragment inserting into a bond. The reactivity depends on the type of carbene complex:

• Fischer carbenes have heteroatom substituents (e.g., $\text{OR}$, $\text{NR}_2$), are electrophilic at the carbene carbon, and are found on low-oxidation-state, late transition metals.
• Schrock carbenes (alkylidenes) lack heteroatom stabilization, are nucleophilic at the carbene carbon, and are typically found on high-oxidation-state, early transition metals.

Carbene insertion reactions include:

• Cyclopropanation: A carbene inserts across a $\text{C=C}$ double bond to form a three-membered ring. The Simmons-Smith reaction (using a zinc carbenoid) is a well-known example.
• C–H insertion: A carbene inserts directly into a $\text{C–H}$ bond, forming a new $\text{C–C}$ bond. Rhodium(II) carboxylate catalysts are widely used for this.

Related processes include nitrene insertion ($\text{M=NR}$) for aziridination and C–H amination, and oxene insertion ($\text{M=O}$) for epoxidation and C–H hydroxylation. These follow analogous mechanistic logic but transfer $\text{NR}$ or $\text{O}$ instead of $\text{CR}_2$.

Activation and Rearrangement Processes

Cyclometallation and C–H Activation

Cyclometallation is an intramolecular process where a metal cleaves a $\text{C–H}$ bond within one of its own ligands, forming a metallacycle. The most common products are five-membered rings, since these have the least ring strain. Nitrogen-donor ligands like 2-phenylpyridine are classic substrates: the nitrogen coordinates to the metal, positioning the ortho $\text{C–H}$ bond close enough for activation. Cyclometallated iridium and platinum complexes are widely used in luminescent materials (OLEDs, for example).

C–H activation is the broader term for any process that cleaves a normally unreactive $\text{C–H}$ bond and replaces it with a $\text{C–M}$ bond. Several mechanisms can operate:

• Oxidative addition: The metal inserts into the $\text{C–H}$ bond (increases oxidation state by 2). Typical for electron-rich late transition metals.
• σ-Bond metathesis: A concerted, four-centered process that exchanges partners without changing the metal's oxidation state. Common for $d^0$ early transition metals and lanthanides that can't undergo oxidative addition.
• Electrophilic activation: The metal acts as an electrophile, abstracting the organic fragment while a base removes the proton. Seen with $\text{Pd(II)}$ and $\text{Pt(II)}$ in acidic media.

C–H activation is a major research frontier because it allows late-stage functionalization of complex molecules without pre-installed functional group handles.

Metathesis and Related Rearrangements

Olefin metathesis redistributes the substituents on alkene carbons. Two alkenes exchange their carbene units through a metal-carbene intermediate, forming new $\text{C=C}$ bonds. The accepted mechanism is the Chauvin mechanism, which proceeds through a metallacyclobutane intermediate:

1. A metal alkylidene ($\text{M=CHR}$) coordinates an alkene.
2. A [2+2] cycloaddition forms a metallacyclobutane.
3. The metallacyclobutane breaks apart in the opposite direction, releasing a new alkene and regenerating a metal alkylidene (with a different substituent).

Major types of olefin metathesis:

• Ring-closing metathesis (RCM): An acyclic diene loses ethylene to form a cyclic alkene. Widely used in natural product synthesis.
• Cross-metathesis (CM): Two different acyclic alkenes exchange partners. Selectivity can be challenging.
• Ring-opening metathesis polymerization (ROMP): Cyclic alkenes open and polymerize, driven by release of ring strain.

Grubbs (Ru-based) and Schrock (Mo- or W-based) catalysts are the workhorses for these reactions. The 2005 Nobel Prize in Chemistry was awarded for the development of olefin metathesis.

Related rearrangements:

• σ-Bond metathesis exchanges ligands at a metal center through a four-centered transition state, without any change in oxidation state. This is the primary bond-activation pathway for $d^0$ metals.
• α-Elimination removes substituents from the carbon directly bonded to the metal, generating a metal-carbene ($\text{M=CR}_2$) complex. This is one route to the alkylidene intermediates needed for metathesis catalysis.